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Flowmeter strain gauge measuring

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Flowmeter strain gauge measuring

Postby AJ_Broady » Thu Jun 16, 2016 11:31 am

Hello,

I'm currently working on a test unit using the TC-08 unit and the TC-08 terminal boards. I am using this to measure the voltage through a strain gauge for a flowmeter thsat my company is currently having problems with.

The strain gauge assembly consists of four bond wires that measure the temperature of the line that the flowmeter is in and convert this to a resistance that can be read and measured by the user, it also consists of two PRTs, one which measure the temperature of the steam passing theough the line that the flowmeter is in and one which measures the temperature of the strain gauge itself.

Currently I have the strain guage set up so that the bond wires and PRTs are being powered by a supply voltage of 5V respectively and having the other two bonds wires terminated in one of the TC-08 terminal boards and the spare wires for both PRTs terminated together in another of the terminal boards.

I have set the PicoLog to take a sample every 100ms and have set each channel to measure mV, the problem I currently have is that when I start the sampling the bond wires will measure correctly and will change accorsing to how I would like (increase and decrease as I move the strain gauge) but the PRTs are not meaasuring as I would expect.
The way the PRTs should react is that the voltage should increase as it detects heat and decrease it is detects cold but it doesnt seem to be detecting/measuring anything.

Could someone plaese help me solve this issue?
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Re: Flowmeter strain gauge measuring

Postby Gerry » Fri Jun 17, 2016 9:09 am

Hi AJ_Broady,

The problem is that a PRT has a very small range over which the resistance will change in response to changing temperature (over a 0°C to 100°C temperature range, a PT100 ranges from 100Ω to 138.5Ω, while a PT1000 ranges from 1000Ω to 1385Ω). So, in order to differentiate these changes with the accuracy, and sensitivity that the sensor provides you need a data logger specifically developed for this, that uses a highly accurate 100Ω resistor reference such as our PT104 (https://www.picotech.com/data-logger/pt ... rature-daq).

It's difficult to visualize what you are doing from your description (a diagram would be better), but your best reduced accuracy alternative would be to use a Wheatstone bridge arrangement, with high accuracy 100Ω resistors (or 1000Ω if you're using PT1000's).

Regards,

Gerry
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Re: Flowmeter strain gauge measuring

Postby Glovisol » Fri Jun 17, 2016 9:17 am

Hi there. As far as I know theTC 08 can accept termocouples, but not Platinum Temperature sensors, because these have a much smaller dynamic range of resistance variation, so you have either to get another measuring unit (e.g. the PT 104) or change the PRT's to another sensor type or build an adapter to match the PRT's to the TC 08.
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Re: Flowmeter strain gauge measuring

Postby AJ_Broady » Tue Jun 21, 2016 8:52 am

Thank you both for your help, I have found the source of the problem and will be testing it shortly to see if the PicoLog will now merasure the strain gauge
Again, thank you both for your help :D :D
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Re: Flowmeter strain gauge measuring

Postby Glovisol » Tue Jun 21, 2016 4:45 pm

Gerry wrote:Hi Glovisol,

A thermocouple is not a very good temperature sensor for the continual good accuracy monitoring required for most flow meter applications, as it has poor repeatability, instability, and reduced accuracy, which is why I was discussing options for using an RTD.

Regards,

Gerry


Dear Gerry,

I totally agree with you, in fact I did not advise the use of a thermocouple, I merely warned Broady that the data logger he was using has been designed (and is fit) for thermocouples and not for platinum sensors...

Cheers,

Gianfranco
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Re: Flowmeter strain gauge measuring

Postby Gerry » Tue Jun 21, 2016 5:36 pm

Hi Glovisol,

Sorry, I was writing a comment for you both but I posted it before it was finished, and then deleted it before I saw your reply.

What I meant to say to you was that the other main reason the TC08 can't work with RTD's is that the RTD needs an excitation voltage which the TC08 can't provide. Also, Building an adpater is an option but is not as straightforward as it sounds because you would need to use a Wheatstone bridge arrangement to avoid losing a lot of accuracy.

Hi AJ_Broady,

In case you were considering it as a solution, a thermocouple is not a very good temperature sensor for the continual good accuracy monitoring required for most flow meter applications, as it has poor repeatability, instability, and reduced accuracy, which is why I was discussing options for using an RTD.

Regards,

Gerry
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